The present invention relates generally to a direct-injection spark-ignition engine having an injector for injecting fuel directly into a combustion chamber, in which a stratified combustible mixture is produced around a spark plug using a tumble during operation by stratified charge combustion. More particularly, the invention pertains to a control device for controlling fuel injection and other factors of such a direct-injection spark-ignition engine as well as to a method of setting fuel injection timing of the same.
In one example of a conventionally known direct-injection spark-ignition engine provided with an injector for injecting fuel directly into a combustion chamber, the air-fuel ratio is increased to produce a lean mixture and the injector injects the fuel during every compression stroke such that the mixture is locally distributed around a spark plug to cause stratified charge combustion in a low-speed low-load operating range, thereby achieving an improvement in fuel economy.
An example of a conventional direct-injection spark-ignition engine is disclosed in Japanese Unexamined Patent Publication No. 2000-104550, in which an intake port is so constructed as to produce a tumble in a combustion chamber and an injector is provided for injecting fuel directly into the combustion chamber. In this construction, the injector injects the fuel in a direction against the tumble in a latter part of the compression stroke during stratified charge combustion so that a fuel spray from the injector hitting against the tumble is carried to an area around a spark plug.
In the direct-injection spark-ignition engine thus constructed, fuel injection time and ignition point during stratified charge combustion are controlled as described in Japanese Unexamined Patent Publication No. 2000-205006, for example. Specifically, the fuel injection time is determined from a predefined xe2x80x9cmapxe2x80x9d according to target load and engine speed while the ignition point is calculated from basic ignition point determined from a predefined xe2x80x9cmapxe2x80x9d according to the target load and the engine speed and various correction values determined according to cooling water temperature and other parameters.
When the fuel injection time during the stratified charge combustion is controlled according to the target load and engine speed as stated above, an arrangement is made to ensure that the period of time from the ending point of fuel injection to the ignition point would not decrease. This is normally achieved by advancing the fuel injection time to increase the difference in crank angle between the fuel injection ending point and the ignition point.
A conventional direct-injection spark-ignition engine using the tumble like the one shown in Japanese Unexamined Patent Publication No. 2000-104550 has a problem that it is difficult to maintain a good stratified charge combustion state over wide engine speed ranges from low-speed to high-speed ranges.
Basically, the higher the engine speed, the higher the velocity of intake air stream, so that a tumble produced by the intake air stream flowing through an intake port into a combustion chamber is intensified as the engine speed increases. Therefore, if penetration of the fuel spray from the injector is constant, the fuel spray tends to be brought to peripheral regions of the combustion chamber in higher speed ranges by tumble flow which becomes more forceful with an increase in the engine speed, even if the engine is designed to properly produce a stratified charge. One approach to the solution of this problem would be to increase the fuel spray penetration to a level corresponding to the intensity of the tumble flow with an increase in the engine speed by increasing the pressure of the fuel supplied to the injector, for example.
Even if the fuel spray penetration is balanced with the intensity of the tumble flow in accordance with the increase in the engine speed in this fashion, however, there arises another problem that the fuel tends to be dispersed in higher engine speed ranges as a result of an increase in hydraulic impact energy.
In the light of the aforementioned problems of the prior art, it is an object of the invention to provide a control device and a method of setting fuel injection timing for a direct-injection spark-ignition engine in which a stratified combustible mixture is produced using a tumble. It is a more specific object of the invention to provide a control device and a fuel injection timing setting method which make it possible to improve fuel economy of a direct-injection spark-ignition engine by causing the engine to make stratified charge combustion in a preferable fashion in stratified charge combustion regions over wide engine speed ranges from low-speed to high-speed ranges.
To achieve this object, a control device of the invention for a direct-injection spark-ignition engine having a tumble generator for creating a tumble in a combustion chamber and a fuel injector which injects fuel into the combustion chamber in a direction opposite to tumble flow such that a combustible mixture is produced around a spark plug at an ignition point in a stratified charge combustion region which is a low-speed low-load operating range of the engine comprises a penetration adjuster for regulating penetration of a fuel spray from the injector in such a manner that it is progressively intensified with an increase in engine speed in the stratified charge combustion region, and a fuel injection controller for controlling fuel injection timing of the injector according to engine operating conditions, wherein the fuel injection controller causes the injector to inject the fuel before the ignition point on compression stroke in the stratified charge combustion region and controls the injection timing such that the interval between a fuel injection ending point and the ignition point becomes shorter with an increase in the engine speed under the same engine load within the stratified charge combustion region.
These and other objects, features and advantages of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.